International Publication Number WO 2013/091094, Organics and Nutrient Recovery from Anaerobic Digester Residues, describes a system in which sludge from an anaerobic digester is treated to recover nutrients. Ammonia in a liquid fraction of the sludge is recovered and used to produce a concentrated acidic ammonium salt solution. This solution may be mixed with a solid fraction of the sludge to produce a nitrogen-enriched solid. Various methods and devices for stripping ammonia are described. In one example, an ammonia stripper has a set of cylindrical reactors arranged for liquid to flow through them in series. Each reactor has a jacket around its outer walls. Hot water flows through the jackets to heat the liquid. A diffuser at the bottom of the reactor introduces air bubbles into the liquid. Gas including ammonia collects in the headspace of each reactor and is removed by connecting a gas outlet to the suction side of a pump.